1. Field of the Invention
The invention relates generally to the field of positioning and leveling arrangements for cables towed, for example, by ships in an ocean environment, and more particularly to paravanes used to position towed arrays streaming behind a towing platform. The invention provides rotational power and control for paravanes.
2. Description of the Prior Art
In, for example, seismic imaging and other similar operations in an ocean environment, sonar devices are generally towed in an array behind towing platforms, such as ships, over an area to be imaged.
The array may comprise a one-dimensional array, in which the sonar devices are attached to a single cable and allowed to stream behind the towing platform. Alternatively, the array may comprise a two-dimensional array, in which a plurality of cables are provided, all of which are to be towed behind the towing platform, and which are to be separated horizontally by selected distances. Positioning devices termed "paravanes" are provided in, in particular, two-dimensional arrays to prevent the cables in the array from converging, which could otherwise occur since the towing force for cables of a two-dimensional array is essentially provided from a single point, namely, the towing platform. Paravanes may also be provided for one-dimensional arrays to facilitate control of the towing depth of the cables in the array, and thus control the depth of the sonar devices below the ocean surface.
Typically, a paravane comprises a body from which two wings extend, with the wings being generally affixed to the body. The body attaches to the cable so that the wings extend outwardly in a plane generally corresponding the longitudinal axis of the attached cable. The plane of the wings is inclined slightly with respect to the longitudinal axis of the cable, which facilitates control of the positioning of the cable relative to the towing platform. The angular position of the paravane with respect to the cable, and thus with respect to the medium through which the cable is to be towed, provides a lateral force which can affect the horizontal position of the cable with respect to the towing direction. By suitably controlling the angular orientation of paravanes on cables in a multiple-cable array, the horizontal separations of the cables in a two-dimensional array can be maintained. The angular orientation of paravanes may be provided by careful control of placement of each paravane on a cable or by suitably weighting one wing or the other. Both of these solutions can present problems, particularly in connection with deploying the array cables.
U.S. Pat. No. 4,711,194, to Fowler, entitled "Streamer Interface Adapter Cable Mounted Leveler" describes a device for controlling the angular orientation of a paravane using an electrical motor. That device, however, includes an appendage that houses the electrical apparatus which is affixed to one side of the paravane. This forms an asymmetric assembly, which also complicates proper orientation of the paravane, and in addition increases the drag of the paravane as the array is being towed through the medium. There is thus a need for an arrangement which is free of these problems.